Conventional microscopes utilizing vertically reflected illumination can not be used to distinguish micron size features in transparent materials where the features have very similar refractive indices. Thus, in order to view features in transparent materials where the features have very similar refractive indices it has been necessary to employ a phase-contrast microscope. A typical phase-contrast microscope is described in section 28.13 of Fundamentals of Optics by Jenkins & White published by McGraw-Hill (1976).
For instance, in the manufacture of wave guides for telecommunication applications, there are features in the wave guides that must be observed so that operations such as laser cutting of inputs and outputs for the wave guides can be completed. In order to position the wave guide for laser cutting it would be necessary to use a phase-contrast microscope to determine the precise cutting regime for the laser. Incorporation of a phase-contrast microscope into a system that determines the cutting regime by using a machine vision system would be cumbersome and result in processing inefficiencies.
Therefore, it is desirable to find alternatives to the phase-contrast microscope when trying to distinguish features having similar refractive indices in a transparent material.